Aberrant regeneration in a case of syringobulbia: selective co-activation of abducens and facial nerves during saccades
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Facial muscles
Abducens nerve
Medial longitudinal fasciculus
Synkinesis
Abducens nucleus
Pons
Tegmentum
Neuroradiology
Reticular connective tissue
One and a half syndrome is an internuclear ophthalmoplegia combined with lateral gaze palsy on the same side. It is caused by ipsilateral lesion of the caudal part of pontine dorsal tegmentum - medial longitudinal fasciculus and paramedian pontine reticular formation or/and abducens nucleus. Usually it is of vascular origin and it can be a sign of lateral pontine haemorrhage. We described a patient with one and a half syndrome caused by spontaneous hypertensive hemorrhage in the lateral pontine legmentum which was visualised by brain CT scanning.
Medial longitudinal fasciculus
Internuclear ophthalmoplegia
Abducens nucleus
Tegmentum
Pons
Abducens nerve
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Pons
Medial longitudinal fasciculus
Abducens nucleus
Pontine nuclei
Oculomotor nucleus
Smooth pursuit
Internuclear ophthalmoplegia
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Five patients with "locked-in" syndrome and dysconjugate palsy of horizontal gaze were studied. In all cases internuclear ophthalmoplegia due to dysfunction or destruction of the median longitudinal fasciculus was combined with an ipsilateral gaze palsy, producing the "one-and-a-half" syndrome. Clinical and electro-oculographic examination suggested involvement of the paramedian pontine reticular formation when all ipsilateral saccades were abolished, when exotropia of the contralateral eye was present, and when vestibular stimulation showed full conjugate deviation to the damaged side. Involvement of the abducens nucleus was suggested when the palsy of ipsilateral gaze was not dissociated on vestibular stimulation. In three cases these clinical deductions were confirmed by the pathological study, which showed a corresponding destruction of the median longitudinal fasciculus, paramedian pontine reticular formation and abducens nucleus. In one case the one-and-a-half syndrome evolved into a total horizontal gaze palsy, which corresponded to involvement of the abducens nucleus contralateral to the initially destroyed paramedian pontine reticular formation. Vertical oculocephalic response disappeared, because of destruction of the median longitudinal fasciculus on both sides (bilateral internuclear ophthalmoplegia). Patients with the locked-in syndrome provide a unique situation in which complex pontine oculomotor disturbances may be studied, because consciousness is preserved. In these patients, dissociated and dysconjugate oculomotor palsy may have been underestimated.
Medial longitudinal fasciculus
Abducens nucleus
Internuclear ophthalmoplegia
Oculomotor nucleus
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The one-and-a-half syndrome is a clinical disorder of extraocular movements characterized by a conjugate horizontal gaze palsy in one direction plus an internuclear ophthalmoplegia in the other. The syndrome is usually due to a single unilateral lesion of the paramedian pontine reticular formation or the abducens nucleus on one side (causing the conjugate gaze palsy), with interruption of internuclear fibers of the ipsilateral medial longitudinal fasciculus after it has crossed the midline from its site of origin in the contralateral abducens nucleus (causing failure of adduction of the ipsilateral eye). Twenty cases are reported 14 had multiple sclerosis.
Medial longitudinal fasciculus
Abducens nucleus
Internuclear ophthalmoplegia
Tegmentum
Oculomotor nucleus
Pons
Abducens nerve
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Abducens nucleus
Abducens nerve
Reticular connective tissue
Oculomotor nucleus
Medial longitudinal fasciculus
Lateral vestibular nucleus
Medial vestibular nucleus
Reticular activating system
Lateral reticular nucleus
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Abducens nucleus
Medial longitudinal fasciculus
Oculomotor nucleus
Abducens nerve
Internuclear ophthalmoplegia
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Horizontal eye movements are conducted by the medial rectus and the lateral rectus muscles, which are innervated by the oculomotor nerve (cranial nerve III) and the abducens nerve (cranial nerve VI), respectively. The oculomotor and the abducens nuclei are interconnected by a tract in the brainstem named the medial longitudinal fasciculus (MLF). Through the MLF, the actions of the oculomotor and the abducens nuclei are coordinated, generating conjugate horizontal eye movements. The disorders of horizontal eye movement that are caused by brainstem lesions are classified into three groups: (a) lateral gaze palsy, (b) internuclear ophthalmoplegia, and (c) one-and-a-half syndrome. Lateral gaze palsy is caused by a lesion involving the paramedian pontine reticular formation (PPRF) or the abducens nucleus. Internuclear ophthalmoplegia occurs as a result of a lesion involving the MLF. One-and-a-half syndrome is a combination of lateral gaze palsy and internuclear ophthalmoplegia and is caused by a lesion involving both (a) the ipsilateral PPRF or the ipsilateral abducens nucleus and (b) the ipsilateral MLF. The pathologic lesions depicted on magnetic resonance images were topographically well correlated with the brainstem pathways and each type of horizontal eye movement disorder. Most of the lesions were tiny acute infarctions and were found in the most posterior region of the pons, which corresponded to the location of the brainstem pathways. Therefore, awareness of the brainstem pathways controlling horizontal eye movement is important to avoid missing a small pontine lesion. © RSNA, 2013
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Medial longitudinal fasciculus
Abducens nucleus
Lateral reticular nucleus
Reticular connective tissue
Oculomotor nucleus
Reticular activating system
Fasciculus
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Abducens nucleus
Internuclear ophthalmoplegia
Medial longitudinal fasciculus
Neuroradiology
Oculomotor nucleus
Pons
Abducens nerve
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I read and viewed with interest the Teaching Video NeuroImage by Kim et al.1 Clearly, the reported patient did not have an "isolated" abducens nuclear infarction, as that would have caused a truly isolated ipsilateral horizontal gaze palsy. The ipsilateral facial palsy was caused by damage to the facial nerve fascicles adjacent to the abducens nucleus. Although the authors indicate this in the body of the text,1 the title of the article suggests that both the horizontal gaze palsy and the facial palsy were caused by the abducens nuclear infarction. I would hate for those who simply read the title of the otherwise excellent vignette to think that facial nerve fibers are somehow located within the abducens nerve nucleus. It is confusing enough for physicians to remember that the nucleus contains both axons destined for the ipsilateral lateral rectus and axons of the medial longitudinal fasciculus destined for the contralateral medial rectus so that abducens nuclear lesions cause an ipsilateral horizontal gaze palsy rather than a sixth nerve palsy.
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